Separation of polymers



Patented Feb. 22, 1949 sEPARA'rroN F POLYMERS John R. Skeen, Philadelphia, Pa., assignor to Sun OiI'Company, Philadelphia, Pa., acorporation of New Jersey No Drawing. Application February 9, 1945,

Serial No. 577,144 L 6 Claims. (Cl. 260-96) The present invention relates to fractionation of polymeric materials with respect to molecular weight and particularly concerns the separation of polymers comprising chain-like molecules of varying molecular weight into'fractions of relatively high and relatively low average molecular weights. The invention especially concerns a method of effecting such separation by selective adsorption on a granular adsorbent such as silica gel.

The method of the present invention is particularly useful in resolving rubber-like polymeric materials into fractions having different average molecular weights and different physical properties. Examples of such materials are isobutylene polymers,-butadiene-styrene copolymers, natural rubber, three-component interpolymers of isobutylene-butadiene-styrene or One method heretofore proposed for making separations of the type herein concerned involves fractional precipitation of the polymeric material from solution. Such method may be carried out by dissolving the polymers in a suitable solvent such as benzol and then adding to the solution an anti-solvent such as acetone or alcohol in amount effective to cause part of polymers to separate from solution. The higher molecular weight polymers, being the least soluble, separate first. By adding incremental portions of anti-solvent and removing the insoluble polymers after each addition, the starting material may be resolved into any desired number of fractions varying in average molecular weight. This method, however, has

proved to be rather impractical particularly for large-scale or commercial use.

According to the present invention, the polymers are separated with respect to molecular weight by selective adsorption on silica gel or other adsorbent material substantially conforming to silica gel in adsorptive properties. This selective adsorption is accomplished by forming a solution of the polymeric material in a suitable solvent such as a saturated hydrocary 2 bon liquid and treating the solution with the adsorbent, preferably by percolating the solution through a bed of the adsorbent. I have found that the polymers of relatively low molecular weight are preferentially retained by the adsorbent while the polymers of relatively high molecular weight tend to remain in solution and pass out along with the solvent.

Further, in accordance with the invention, the polymer fraction retained by the adsorbent is removed therefrom by treating the adsorbent with a primary desorbing agent which is sad!- ciently adsorbable to displace the polymer therefrom but which leaves the adsorbent in an inactivated state due to adsorbed primary agent. Examples of suitable primary agents are benzene and ethylene dichloride. The primary agent, in turn, is displaced by treating the adsorbent with a secondary desorbing agent which has a relatively low degree of adsorbab li such as a saturated hydrocarbon or other organic liquid substantially conforming thereto in adsorbability properties, thereby to reactivate the adsorbent and condition it for further use. Heptane is one, example of the type of organic liquid suitable as the secondary desorbing agent.

For practicing the desorbing steps of the process, the primary desorbing agent should be sumciently strongly adsorbable to be capable of displacing the adsorbed polymers but not so strongly adsorbable as to be incapable, itself, of displacement by means of the secondary desorbing agent. I have found that organic liquids having the properties hereinafter set forth meet these requirements and are suitable primary desorbing agents.

In orderthat the primary desorblng agent be capable of removing the polymers, it should be "more strong adsorbable" by the particular adsorbent used in the process than are the polymers which it is to displace. This may be determined by means of the adsorption isotherm for a binary mixture of the primary agent and the polymers. If the adsorption isotherm shows that an organic liquid is preferentially adsorbed from binary mixture with the polymers throughout more than 50 per cent of the concentration range, it may be said, for the present purpose, that it is more strongly adsorbable" than the polymers and therefore is capable of displacing them. It, accordingly, would meet the first requirement for' a suitable primary desorbing agent.

The second requirement, namely, that theprlmary desorbing agent be not so strongly adsorbable as to be incapable of being displaced-by the secondary desorbing agent, may be determined from the adsorption isotherm for a binary mixis herein termed the adsorption index of the primary agent. It has been found that inorganic liquids which have an adsorption index" not ex- I ceeding 40 are capable of being displaced by the secondary desorbing agent. Organic liquids which have "adsorption indexes within the range of about '7 to about 20 give best results and are preferred as the primary desorbing agent.

As used herein in both the description and claims, the adsorption index" of a compound may be defined as the number of cubic centimeters of thecompound adsorbed by one kilogram of the adsorbent when the latter is in equi-' 'librium with a solution consisting of 0.2 per cent by volume of the compound and 99.8per cent by volume of the secondary desorbing agent employed in the process. is to be used as the adsorbent and pentane as 'thesecondary desorbing agent and it is desired to determine whether or not benzene would be a satisfactory primary desorbing agent, then the For example, if silica gel gives values which have been obtained for several pairs of organic liquids:

t Adsorption Primary Liquid Secondary Liquid Riggs;

' Liquid mcthylcy n-hep 0.? carbon tetrachloride.-- 0.8 8 do. 18 methylcyclohcxane--. 68 (10 160 methyl alcohol -do 196 The relatively low value of 0.2 for methylcyclohexane in n-heptane indicates that the difleren'ce in adsorbabilities of naphthenes and of paraffins, as exemplified by these two specific members of these hydrocarbon-types, is negligible insofar as the present process is concerned. Either methylcyclohexane or n-heptane is a good example of the secondary desorbing agent. Other examples are n-butane, isobutane, n-pentane,

adsorptionindex of benzene would be determined as the number of cubic centimeters of benzene adsorbed by one kilogram of silica gel would indicate that there is no difference in ad-.

sorbabilities of the two materials. As the adsorption index increases, the amount of secondary agent required to effect displacement of the primary agent increases. When the adsorpisopentane, cyclopentane, isohexane, n-hexane, cyclohexane, octanes, etc., or mixtures of such hydrocarbons suchas petroleum ether. In fact,

- any saturated hydrocarbon which is liquid under the conditionsof operation could be used as the secondary desorbing agent. Furthermore, some organic liquids other than hydrocarbons are suitable as the secondary desorbing agent. For example, carbon tetrachloride. which, as shown above, has an absorption index in n-heptane of only 0.8, has an absorbability substantially equivalent to that of saturated hydrocarbons and therefore may be used as the secondary desorbing agent. For the present purpose, it may be considered that any organic liquid which, in binary so- .lution with a saturated hydrocarbon such, for

g a solvent for the starting polymers.

tion index has a value above about 40, the adsorbed primary agent can be displaced only if an unreasonably large amount of the secondary agent is used. I I

A convenient method of ascertaining the "adsorption index" comprises obtaining several points on the adsorption isotherm for a mixture of the primary and secondary agents at concentrations below 5 per cent primary agent and then extrapolating or interpolating to an equilibrium concentration. of 0.2 per cent primary agent.

- Methods of determining the adsorption isotherm within any desired concentration range are well Benzene has been found to bemore strongly I absorbable than long-chain polymers and therefore'capable of desorbing them. Since benzene hasan adbsorption index of 8 as shown above, it is also capable of being, in turn, displaced by I means ofthe secondary type of desorbing agent.

Benzene is thus an excellent primary desorbing agent. Toluene, xylene and other benzene derivatives likewise are suitable primary agents.

Ethylene dichloride, which also is more strongly 'absorba'ble than long-chain polymers and which has an absorption index of 18, is another example of a very satisfactory primary agent.

Other halogenated hydrocarbons, including iodine, bromine and fluorine derivatives likewise are satisfactory. However, halogenated hydrocarbons which have a symmetrical molecular structure, such as carbon tetrachloride, in general are too weakly adsorba-hle to displace the polymers and are not suitable as the primary agents but rather have the properties required of the secondary agent.

Ethyl ether is too strongly adsorbable, as shown by an adsorption indexfof 68, to be displaced by reasonable amounts of the secondary agent and therefore. is not a suitable primary agent.

Acetone and methyl alcohol have such high indexes that, for all practical purposes, it may be considered that they are incapable of being disflaeed bymeansofthesecondarymot s oxy-hywhicharenottoohighlypolarmaymeetthe requirements tor a satisfaetm'y primary desorbent The following serve to illustratespecific embodiments of the invention and are given merely by way of illustration:

Emaplcl 'l'hestartingmaterlalwasavulcanizablerubber-likepolymerlcmaterialwhichhadbeenpreparedbyint rp lymerizingamixturecomprlslng sopartsot isobutylene,30partsofbutadieneand Approximately 500 mi. of the first filtrate, representing most of the liquid hold-up of the column.

'wasdiscardedaiterwhichninefiltratecutsas tabulatedbelow were taken. The polymer fraction was obtained from each cut by evaporating parts of styrene at a temperature of C. to ii the solvent.

Polymer Fm (solvent free) on N6 Vol. of Solvent in Penetration Wt. Per cent Bolinctivo Specific Brunino oi Chg. ASTM ll-M Index Disp. No.

Method 1 Method i 15 penbme- 8.2 0 1 1.515 131 62.2 110 I10 11. 9 2. 5 1. 5!!) 131 110 fin ll. 7 6 1.5% 131 110 do 11. 5 10 1.5293 131 110 do 12.5 11 1.520 131 62.1 110 do 11. 1 l 11. 5 1.5280 131 110 .do 8.0 19 1.5255 1:0 .3 125 8. l 450 1.5108 12) 1, (III do- 11.0 (fluid) 1. 51W 1D 73. 2

gram weight for 5 C.bymeans of4partsofAlChusedinthe form of a slurry in ethyl chloride. The starting The tabulated values for penetration show that the starting material was separated into fractions polymers had a penetration of 212 (as'rm varying widely in degree of hardness, ranging method D-5-25). Five hundred grams of the polymers were dissolved in 2000 ml. of pentane and the mixture was percoiated through 1800 grams of silica gel in a column 2 ,5 inches in diameter and 30 inches in height. Polymers of relativeb low molecular weight were adsorbed by the gel whereas those of relatively high molecular from a very hard, brittle product to even a fiuid, oily product. p The small variations in specific dispersion. and bromine number indicate that the polymers were relatively uniform in chemical composition (i. e., the variousfractions contained approximately the same proportions of aromatic rings and unsaturate groups) and that the fractionation was eifected primarily in accordan with molecular weight.

Example III tion wherein the used adsorbentis regenerated and properties of the were as follows: f and reused.

Yield Grams 5 g Appearance High molecuhr weight s15 $3.0 V a hold, m Low molecular weight traction--- 168 33.6 an m salt, lucky Les 11 a4 The difference in average molecular wel ht ot the fractions is clearly indicated by the large diiference in penetration values.

Example I! The present example illustrates the separation of polymersintoaseriesoffractions ofvarying 05 Example I. It had a penetrationof 82 and a The starting material was a rubber-like interpolymer product formed by polymerizing 40 partsisobutyle'ne; 40 parts butadiene and 20 parts styrene with the temperature and catalyst as in bromine number of 54. A column 14 inches in diameter and 40 inches hi h packed with lbs. of 28-200 mesh silica gel was used forseparating a ma! of 480 lbs. of the startin material into two fractions which differed in molecular weight and hardness. In each cycle of operation, a solution consisting of 40 lbs. of the starting material dissolved in 18% gallons of isopentane was percolated through the gel, after which 24 gallons of toluene were passed therethrough to desorb the relatively low molecular weight polymers which had been retained. This was followed by 38% gallons of isopentane to displace the adsorbed toluene and restore the activity of the gel. This average molecular-weights which comprises formprocedure was followed throughout 12 cycles.

fractions from each cycle were combined. Yields and properties of the fractions were as follows:

Yield Per cent Penefiw Lbs. mm

Appearance Charge 7 High mol. wt. 30

traction.

Low mol. wt.

fraction. l

resinous, amber cole viscous oily fluid.

The present invention is not necessarily limited to the Use of silica gel as the specific adsorbent,

The solvents were removed from the two polymer fractions by evaporation and the corresponding since other adsorbents conforming substantially to silica gel in adsorptive properties also may be utilized. Various activated clays, although gen-, erally having less eflicient adsorptive properties: than silica gel, may meet the requirements satisfactorily. The primary desorbing agents suitable for use with other desorbing agents will, in gengeneral, be those employed with silica gel, al-

though they will not necessarily be the same in all cases. In choosing a primary desorbing liquid 1 for use with a specific adsorbent, it is only necessary to determine that the liquid in question 7 meets the two requirements, specified above, of being more strongly adsorbable by the particular adsorbent used than is the polymer fraction which is to be displaced, and of having an a sorption index not exceeding 40.-

In choosin desorbing agents for practicing the process, it is desirable that they be so selected with respect to boiling point that they may be easily separated from the products and from each other by distillation. A course, should have no tendency to react (e. g.

The desorbing agents, of

polymerize) in the presence of the adsorbent under .the operating conditions. What I cfaim and desire to protect by Letters Patent is:

1. Method of separating polymers comprising chain-like molecules of varying molecular weight into fractions of relatively high and relatively ing a solution of said polymers and a saturated hydrocarbon solvent, contacting the solution with silica gel to selectively adsorb polymers of relatively low molecular weight while leaving relatively high molecular weight polymers in solution, separatingfrom the adsorbent the treated solution containing the high molecular weight fraction and washing the used adsorbent with a desorbing agent to remove the relatively low molecular weight fraction.

' 3. Method of separating polymers comprising chain-like molecules of varying molecular weight into fractions of relatively high and relatively low average molecular weights which comprises formin a solution of said polymers and a saturated hydrocarbon solvent, contacting the solution with silica gel to selectively adsorb polymers of relatively low molecular weight while leaving relatively high molecular weight polymers in solution, separating from the adsorbent the treated solution containing the high molecular weight fraction, washing the used adsorbent with an organic primary desorbing agent to remove the relatively low molecular weight fraction, said primary desorbing agent being more strongly adsorbable than said low molecular weight polymers but not so strongly adsorbable as to have'an adsorption index exceeding 40, and then washing the adsorbent with a saturated hydrocarbon liquid to effect desorption of the primary desorbing agent and thereby reactivate the adsorbent.

4. Method 'deflned in claim 3 wherein the primary desorbing agent is a hydrocarbon of the henzene series.

5. Method defined in claim 3 wherein the primary desorbing agent is ethylene dichloride.

6. Method of separating polymers comprising chain-like molecules of varying molecular weight into fractions of difierent average molecular weights which comprises forming a solution of said polymers and a. saturated hydrocarbon solvent, percolating the solution through a bed of silica gel to selectively adsorb a portion of the polymers while leaving in solution polymers of higher average molecular weight than those adsorbed and separating from the adsorbent the treated solution containing the polymers of higher average molecular weight.

JOHN R. SKEEN.

REFERENCES crrnn The following references are of record in the file of this patent: 1

UNITED STATES PATENTS Number Name Date 2,316,774 Dunmire et al Apr. 20, 1943 2,375,596 Strickland ay 1945 OTHER REFERENCES Laboratory Technique in Organic Chemistry, by A. A. Morton, pub. 1938 by McGraw-Hill Book Co., Inc., New York, pages 187-188, 

